The interaction of matrix screw dislocations with twin boundaries has been simulated by computer for different h.c.p. models representing a-titanium and magnesium. The atomic structure of the screw core in these two models is appropriate for crystals that slip predominantly on the prism and basal planes, respectively, and this behaviour is summarised in the first part of the paper. Then the movement under three different components of applied strain of the 1/3< 11–0> matrix screw dislocation (...

The interaction of matrix screw dislocations with twin boundaries has been simulated by computer for different h.c.p. models representing a-titanium and magnesium. The atomic structure of the screw core in these two models is appropriate for crystals that slip predominantly on the prism and basal planes, respectively, and this behaviour is summarised in the first part of the paper. Then the movement under three different components of applied strain of the 1/3< 11–0> matrix screw dislocation (in both its prism and basal forms) into the boundary of the 10–12 and 10–11 twins is described for the geometry where the screw is parallel to the interface. The screw crosses the 10–12 boundary by cross-slip, onto either of the two slip systems, but the 10–11 boundary usually absorbs the screw by a process of decomposition into two twinning dislocations. This behaviour and the glide resistance are discussed in terms of the interfacial structure of the twins and the properties of twinning dislocations.